Fresh food is wrapped automatically on processing lines with cling wrap film. The procedure involves the pulling off at high speed of film from a supply roll intermittently, pushing a tray filled with food upward until is impacts against the film, stretching the film over the sides of the tray to the lower side thereof, heat sealing (without melting) the edges of the film, and allowing the film to be pulled taut by elastic contraction. In use the film must be transparent, puncture resistant and must snap-back when deformed. This procedure has been performed satisfactorily with PVC. However it has been difficult to formulate compositions based on olefinic polymers for making cling wrap film. Any such composition must not only possess characteristics suitable for processing in the manner described above, but also be approved for food law purposes. The characteristics which make a film suitable for such automated processing may be difficult to relate to specific physical parameters; a balance of measurable properties only gives a general indication of suitability. U.S. Pat. No. 4,657,982 describes a film useful for stretch wrap applications. Stretch wrap film is used for example for holding packages together on a pallet. Such film is applied stretched and the holding force is provided by the tendency of the film to return to its original size elastically. Such films hence require a different balance of properties from food wrap film of the kind to which the invention relates. Stretch wrap films have a high elongation of 150% or 200% or over and tear propagation, once tearing is initiated, is very fast. Hence Elmendorf tear resistance is often used to measure tear strength. Trouser tear strengths are low for stretch wrap film.
The film of U.S. Pat. No. 4,657,982 includes a high proportion of strength providing polyethylene (from 80 to 99%); a small proportion of a tackifying low molecular weight polybutylene; and a small proportion of a polybutylene having a number average molecular weight of from 1 700 to 10 000. Up to 30% of the polyethylene may be EVA. There is no disclosure of a high EVA blend containing more EVA than polyethylene.
The low molecular weight polybutylene has a tackifying effect without adding to the film strength or to elastic properties.
U.S. Pat. No. 4,425,268 discloses in Example 1 a stretch wrap blend of 30% of LLDPE, 68% of EVA (12% VA), 1% of a low molecular weight polyisobutylene (PIB) and 1% of glycerol mono-oleate (GMO). No elastomer component was added. The PIB is added to provide sufficient sealing force without heating. GMO probably acts as agents for facilitating release of the film from the roll. U.S. Pat. No. 4,504,434 describes a similar material also for stretch wrap applications.
EP 80198 uses homopolymer and copolymer with low molecular weight PIB to provide cling or tack. Vinyl acetate is mentioned as a possible comonomer. The film contains no elastomer component and has a high tensile strength.
EP 92318 discloses a broad range of compositions incorporating for example LLDPE (linear low density polyethylene) as a polyolefin plastic component; EVA as component of a plasticiser; an olefinic elastomer (polybutylene is mentioned); and a hydrocarbon oil. EP 92318 does not disclose the specific combination of LLDPE, EVA and PIB and the incorporation of at least 1% of the hydrocarbon oil is required. The exemplified films are fairly stiff and highly elastic (the 1% secant modulus is generally above 25 000 psi).
A variety of blends have been proposed to meet specific food wrap requirements. However these blends have not replaced PVC substantially for automated processing lines as far as Applicants aware. The following documents discussed show clearly the range of proposed solutions.
U.S. Pat. No. 3,932,563 describes a material containing over 90% of EVA and uses low molecular weight PIB and no LLDPE. EP 11930 is similar.
EP 66149 uses a mixture of high pressure LDPE and low pressure LDPE and a cling agent such as glycerol oleate. There is no EVA or elastomer component present. The material will have low stretchability and will be prone tearing on high speed automatic processing lines. EP 65359 is similar.
GB 2123747 suggest the use of two different layers in the film one being an LLDPE-EVA blend, the other being a high EVA-ionomer resin blend.
GB 2094235 discloses a packaging film (which is probably a stretch wrap) containing a high amount of EVA (over 90%) and an ethylene propylene copolymer for frozen food packaging.
U.S. Pat. No. 4,500,681 discloses a blend containing a polypropylene olefin component, EVA and from 20 to 60% of an isobutylene--backbone elastomer component. The high isobutylene component content and the polypropylene would yield a material unsuited as a highly transparent cling film.
It is the object of the invention to provide cling film suitable for food wrapping which has a low yield strength, a low 1% secant modulus, a high impact resistance and high puncture resistance.
It is also an object of the invention to provide a cling wrap film suited for automatic processing to be used as a substitute for PVC.